1. Field of the Invention
This invention relates to an electropneumatic clutch control device for motor vehicles, primarily industrial vehicles.
2. Description of the Background Art
The most widely used system, mainly on light vehicles, is the unboosted control in which the clutch is connected to the pedal by a rod, cable or hydraulic linkage with emitter and receiver. On heavy trucks very frequently the control is boosted pneumatically supplemental to a mechanical or more generally a hydraulic linkage: this is the clutch servo.
These different systems have numerous drawbacks, particularly for pedal force, progressiveness and maintenance.
Force on the pedal represents a proportional mechanism control force, but it is increased during declutching or reduced during clutching by the sum of all the frictions from each of the components, which produces a significant declutching force and significant variation in force between declutching and clutching.
The clutch has three modes of use: when shifting gears, when starting up and when it is not operated.
When shifting gears, the clutch is operated all or nothing, and above all must ensure good disengagement between the engine and transmission so as not to damage the synchronizers. The mechanism is thus designed so that its control ensures "plate lift," by correct travel.
When starting up, the clutch is operated with "controlled torque extraction," i.e., to ensure progressive startup, the driver gauges pedal movement to "make it slip."
From then on it is felt that the more significant the slippage, the more progressive the clutching will be judged. In addition to the declutching or gear shifting phases, the clutch is not operated and its main role then is to transmit engine torque to the transmission (or reciprocally) with no ill-timed slippage. The clutch thus provides, when not operated, nominal preslippage torque, much greater than engine torque.
In short, the clutch mechanism control has three areas, each with its own function:
a travel corresponding to clutch "overcalibration,"
"slippage" travel useful in startups,
and a "plate lift" travel which is needed for shifting gears.
The problem with present controls is, paradoxically, their excessive linearity. With a linear control, the same proportion of slippage travel is reproduced with the pedal as in total travel. Now, with the usual clutches, this proportion is rather small, which produces progressiveness that is not ideal unless the overall travel is increased, with the resulting loss of comfort.
Lastly, the known systems such as hydraulic linkage systems have the drawback of requiring constant maintenance and posing ticklish bleeding problems.